This invention relates generally to fuel cells, and more particularly to methods and systems for humidifying a supply of fuel for use in a fuel cell fuel processor.
Fuel cells electrochemically convert reactants, for example, a fuel and an oxidant, to electricity. Unlike batteries, which typically contain a set amount of chemicals for generating electricity and which stop delivering electricity once the chemicals are consumed, fuel cells can deliver electricity continuously as long as the fuel cells receive a fuel and an oxidant. Fuel cells are generally categorized according to the type of electrolyte (for example, solid oxide, molten carbonate, alkaline, phosphoric acid, or solid polymer) used to accommodate ion transfer during operation. For example, a solid polymer electrochemical fuel cell generally comprises a membrane electrode assembly (MEA). The MEA includes a solid polymer membrane or proton exchange membrane (PEM) sandwiched between and in contact with two electrodes: one electrode is referred to as the anode and the other electrode is referred to as the cathode. The membrane is typically made of a porous, electrically-conducting sheet material. The electrodes are typically made from carbon-fiber paper or cloth. In addition, at the interface of the electrode and membrane, that is, sandwiched therebetween, a platinum-based catalyst layer is provided to facilitate the electrochemical reaction.
Typically, the MEA is placed between two electrically conductive graphite plates, which have one or more reactant flow passages impressed on the surface. The reactant flow passages direct the flow of reactants to the membrane electrode assembly.
Fuel, such as humidified hydrogen, is supplied to the anode side of the fuel cell where the hydrogen reacts at the platinum-based anode catalyst layer to separate into hydrogen ions and electrons, as follows (the anode reaction):
H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83(1)
The solid polymer membrane permits the passage of protons (that is, the H+ ions) from the anode side of the fuel cell to the cathode side of the fuel cell while preventing the passage therethrough of reactants (for example, hydrogen and air/oxygen gases). The electrons (exe2x88x92) generated in the reaction of Equation 1 pass through an external circuit in the form of electricity. Oxidant, such as humidified oxygen or air, is supplied to the cathode side of the fuel cell. In this cathode side, the oxygen reacts, in the presence of the platinum-based cathode catalyst, with the hydrogen ions that have passed through the membrane and with the electrons that passed through the external circuit to form liquid water, as follows (the cathode reaction):
1/2O2+2H++2exe2x88x92xe2x86x92H2Oxe2x80x83xe2x80x83(2)
Additional cells can be connected together in series to form a xe2x80x9cfuel cell stackxe2x80x9d which provides the cumulative voltage and power output of the individual cells. Such a fuel cell stack is typically provided with inlets, outlets, and manifolds for directing the flow of reactants (as well as coolant, such as water) to the individual reactant flow plates, and assembled between a pair of thick rigid end plates. The edges of the end plates are bolted together to apply a compressive force on the fuel cell stack.
In PEM fuel cell systems, typically the membrane works more effectively if it is wet. Conversely, once any area of the membrane dries out, the electrochemical reaction in that area stops. Eventually, the dryness can progressively march across the membrane until the fuel cell fails completely. As a result, the fuel and oxidant fed to each fuel cell are usually humidified, e.g., with steam.
Where pure reactants are not readily available or economical to supply to a fuel cell, it may be desirable to use air as an oxygen source, and to use a fuel processor to convert a hydrocarbon such as methane or methanol into a hydrogen rich stream. The two reactions which are generally used to achieve this conversion as shown in equations (3) and (4).
1/2O2+CH4xe2x86x922H2+COxe2x80x83xe2x80x83(3)
H2O+CH4xe2x86x923H2+COxe2x80x83xe2x80x83(4)
The reaction shown in equation (3) is sometimes referred to as catalytic partial oxidation (CPO). The reaction shown in equation (4) is generally referred to as steam reforming. A fuel processor may use either of these reactions separately, or both in combination. While the CPO reaction is exothermic, the steam reforming reaction is endothermic. Reactors utilizing both reactions to maintain a relative heat balance are sometimes referred to as autothermal (ATR) reactors. Also, it should be noted that fuel processors are sometimes generically referred to as reformers, and the fuel processor output gas is sometimes generically referred to as reformate, without respect to which reaction is employed.
As evident from equations (3) and (4), both reactions produce carbon monoxide (CO). Because of the high temperature at which the fuel processor is operated, this CO generally does not affect the catalysts in the fuel processor. However, if this reformate is passed to a fuel cell system operating at a lower temperature (for example, less than 100 degrees C.), the CO may poison the catalysts in the fuel cell by binding to catalyst sites, inhibiting the hydrogen in the cell from reacting. In such systems it is typically necessary to reduce CO levels to less than 100 ppm. For this reason the fuel processor may employ additional reactions and processes to reduce the CO that is produced. For example, two additional reactions that may be used to accomplish this objective are shown in equations (5) and (6). The reaction shown in equation (5) is generally referred to as the shift reaction, and the reaction shown in equation (6) is generally referred to as preferential oxidation (PROX).
xe2x80x83CO+H2Oxe2x86x92H2+C2xe2x80x83xe2x80x83(5)
CO+1/2O2xe2x86x92CO2xe2x80x83xe2x80x83(6)
As evident from equations (4) and (5), water may be employed as a reactant in a fuel processing system. For example, a fuel may be humidified with steam prior to entering the fuel processor, or liquid water may be fed into a desired reactor within the fuel processing system. It thus may be desirable to control the amount of water added to the fuel processor, for example to control the temperature of an ATR or reforming reactor, or to drive the shift reaction to eliminate carbon monoxide. In the case of ATR and reforming reactors, the amount of water in feed streams to such reactors is generally referred to as the steam-to-carbon ratio.
The present invention generally comprises methods and systems for increasing the efficiency of fuel processor systems. This is accomplished by humidifying a fuel stream used in a fuel cell prior to pressurizing the fuel stream. In particular, fuel having a lower pressure can be humidified with a humidifying liquid, such as water, with less energy compared to the energy required to humidify the fuel after it is pressurized.. As an example, less energy need be expended heating the humidifying liquid.
In one aspect of the present invention, a method for forming a hydrogen-rich stream (also referred to as reformate) for use in a fuel cell includes combining water and at least one of fuel and air to form a mixture at a first pressure, pressurizing the mixture to a second pressure greater than the first pressure, and reacting the mixture to produce a hydrogen-rich stream.
In a second aspect, a system for forming a hydrogen-rich stream for use in a fuel cell includes a pressurizing device having an inlet for receiving a mixture of water and at least one of fuel and air at a first pressure and an outlet for discharging the mixture at a second pressure greater than the first pressure, and a reactor having an inlet operatively connected to the outlet of the pressurizing device and an outlet for discharging the hydrogen-rich stream for use in the fuel cell.
In a third aspect, a method for forming a hydrogen-rich stream for use in a fuel cell includes (a) providing a first stream of fuel having a first moisture content, (b) exposing the first stream of fuel to an aqueous fluid to produce a second stream of fuel at a first pressure and a second moisture content higher than the first moisture content, (c) pressurizing the second stream to produce a third stream of fuel having a second pressure higher than the first pressure, and (d) introducing the third stream to a reactor to produce the hydrogen-rich stream.
In a fourth aspect, a method for forming a hydrogen-rich stream for use in a fuel cell includes heating water, combining the heated water with at least one of fuel and air to form a mixture at a first pressure, pressurizing the mixture to a second pressure greater than the first pressure, and reacting the mixture to produce the hydrogen-rich stream.